Back-scattering detection systems, including frequency modulated continuous wave (FMCW) radars and radio frequency identifications (RFID) readers, simultaneously transmit an RF carrier wave and receive backscattered radio waves through their antennas. However, the transmitted and received signals for a transceiver may overlap in terms of frequency spectrum, as the received signals may have a frequency separation too small to be easily filtered. Therefore, the leading transmitted signal becomes a potential blocking signal to the back-scattered signal at the receiver front-end, as the back-scattered signals from the target object may be much smaller in amplitude than the transmitted signals.
One type of back-scattering detection system includes a bi-static RFID reader. Bi-static RFID readers have separated transmitting and receiving antennas. The transmitting circuitry (e.g., a modulator, power amplifier, transmitting antenna, etc.), does not share the same physical signal path with the receiving circuitry (e.g., a demodulator, low noise amplifier, receiving antenna, etc.). Physical separation between transmitting and receiving antennas may be employed to reduce the leakage of transmitting carrier to the receiving front-end RF circuitry.
Another type of back-scattering detection system includes mono-static RFID readers, which use one antenna for both transmission and reception. It is desirable to keep the transmission leakage at a low level in a mono-static back-scatter reception system since the simultaneous operation of transmitter and receiver is enabled by a shared antenna. In a mono-static RFID reader a circulator may be used to determine the direction of the transmitting carrier wave and arriving backscattered radio signal for detection. Alternatively, directional couplers may also be used in lieu of circulators due to their cost advantage. Therefore, the antenna, the coupler (or the circulator), and connecting cables therebetween are preferably shared by the transmitter and receiver.
Unlike bi-static readers, whose leakage factors are mostly determined by the isolation of their antennas, the transmission leakage at the receivers of a mono-static RFID reader is mostly limited by the imperfect reflection coefficient of the shared antenna, transmission line, coupler, terminator, and/or circulator. Therefore, a higher level of transmission leakage is likely to be observed on a mono-static reader.
The transmission to reception leakage of a directional coupled based mono-static reader is affected not only by the fixed components of the RF hardware, but also by the variable RF components including the temperature-related component variation, the bend or twist of RF cables and components, the degradation of wire connection due to deformation or oxidization, or any other damage of components due to the environment. Moreover, any movement of objects near the antenna at the installation site also affect the impedance of antenna, thereby further altering the transmission to reception leakage ratio. The environmental effect of the mono-static RFID reader is a significant source of degradation of the transmission to reception leakage.
To overcome the forgoing disadvantages, various embodiments described and/or suggested herein preferably include circuitry to reduce the transmission leakage at the receiver, e.g., in back-scatter detection radio systems.